When NeverTrump Sen. Jeff Flake, R-Ariz., announced he would not seek re-election in 2018, nearly everyone knew it was because his anti-Trump positions made him unelectable in his upcoming Republican primary.

Three candidates are vying in the Republican Senate primary to replace Flake; two are attempting to be the most pro-Trump candidates, and the front-runner is backed by the Republican establishment.

Rep. Martha McSally, R-Ariz., is the front-runner and is widely expected to win the primary. She was first elected to the House in 2014 and previously served in the Air Force, retiring in 2010 as a colonel. She was the first female fighter pilot to fly in combat and has made her military background a centerpiece of her campaign. The Senate Leadership Fund, a super PAC affiliated with Senate Majority Leader Mitch McConnell, R-Ky., has supported her candidacy.

McSally faces former state Sen. Kelli Ward, who unsuccessfully challenged the late Sen. John McCain, R-Ariz., in the 2016 Republican Senate primary. She has been endorsed by several conservative lawmakers including Reps. Paul Gosar, R-Ariz., Steve King, R-Iowa, Rep. Dana Rohrabacher, R-Calif., and Sen. Rand Paul, R-Ky.

Throwing a wrench in the race is former Maricopa County Sheriff Joe Arpaio, who in 2017 was found guilty of criminal contempt for his decision to disregard a 2011 court order. Arpaio was pardoned by President Donald Trump later that year and tossed his hat into the Senate race running on Trump’s support. He has emphasized immigration enforcement and support for Trump’s immigration policies.

Arizona is a closed primary state, meaning only voters registered with a political party may vote in that party’s primary election. Voter registration closes 29 days before the primary election. The polls will close at 10:00 p.m. ET.

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